Nowadays, the mainstream wireless charging products in the market uses the coiled metal wire to transfer the energy. Most of the products reach the specific operation frequency by adjusting the coil size or the coiling number of the metal wire coil. As shown in FIG. 1, the wireless charging system 9 includes a coupling coil 91 and a matching circuit 92. The coupling coil 91 provides inductance (L), and the matching circuit 92 provides capacitor (C) to form a LC series resonance to increase the coupling energy efficiency. In addition to provide the desired electric capacity for resonance, the matching circuit 92 also has an impedance conversion function. The inductance and capacitance value are both important parameters to control the operation frequency.
Generally, the matching circuit 92 utilizes the lump-element, such as the ceramic capacitor. The lump-element includes series capacitor (C1) and parallel capacitor (C2). But the lump-element has inherent limitations in the usage of matching circuit 92. For example, the capacitance value of every single lump-element is fixed, and the adjustable capacitance values are not continuously between each other. Thus, there are no suitable capacitance values that can be fitted so as to the wireless charging system 9 is unable to achieve the best design. Although users can utilizes a number of capacitors in parallel to increase the design flexibility, but the increase of the capacitance also increases of the occupancy area, the design time and the production cost of the matching circuit 92. With smaller and smaller capacitance value, the percentage error caused by the lump-elements will get bigger and bigger. Therefore, the present invention is considered necessary to provide a new wireless charging circuit to improve the above problems.